The present invention relates to a high carbon chromium bearing steel which is used as a material for bearings having extremely outstanding rolling fatigue life.
The rolling fatigue life of a rolling bearing generally ends with the flaking of its raceway surfaces which are in a rolling contact with rolling elements. There are, however, two types of such flaking, that is, surface origin flaking originating in damage caused on the surface during use, and internal origin flaking originating in non-metallic inclusions present near the point of maximum shear stress below the aforementioned raceway surfaces. In order to prolong the rolling fatigue life (hereinafter simply "life"), hardness is increased to make it harder to scratch the surface, thereby prolonging the life with respect to the aforementioned surface origin flaking, and the content of non-metallic inclusions is reduced to reduce sources of cracks, thereby increasing the life with respect to the forementioned internal origin flaking. Furthermore, samples (Japanese patent laid-open no. 61-272349) in which the life has been prolonged by limiting the sulfur and phosphorus content, both of which are unavoidable impurities, to less than 0.010% by weight, and samples (Japanese patent laid-open no. 60-194047) in which the life has been prolonged by adding silicon in an amount of 0.15% to 1.6% by weight are known.
Conventionally, high carbon chromium bearing steels used for rolling bearings with consideration given to the aforementioned problems include high carbon chromium bearing steels (SUJ1 to SUJ5) as specified in Japan Industrial Standards (JIS) G4805. Of these, SUJ2 s most common. This SUJ2 steel is characterized by carbon in an amount of 1% by weight and chromium in an amount of 1.5% by weight; the structure of this steel is martensitized in the quench and temper heat process, and SUJ2 is generally used with approximately 10% residual austenite and approximately 7% non-solute carbide content. A hard coating is also provided in certain circumstances.
However, insofar as the aforementioned SUJ2 metal is used, there is a limit to the hardness which relates to the aforementioned surface origin flaking. Furthermore, when a hard coating is applied to the surface, the interfacial strength between the coating layer and the matrix is low. Furthermore, with respect to the internal origin flaking, we have almost approached the limit to the reductions which can be realized in non-metallic inclusions using current production methods, and the prospects for further significant reductions in non-metallic inclusion content are not good.